Lithium, usually in the form of lithium carbonate, is commonly used for the treatment of manic and manic depressive patients. Although traces of lithium ion are distributed widely throughout the body, the major effect of exogenous lithium is upon the central nervous system (CNS). Unlike other antipsychotic pharmaceutical agents, lithium is not thought to possess any general sedative properties, and, in therapeutic amounts, CNS effects are not observed except during chronic administration of lithium in manic or manic-depressive patients. The exact therapeutic mechanism of lithium is not currently known, primarily because the pathophysiology of manic disorders is unknown.
Lithium ions are readily absorbed when given orally, and a plasma lithium peak is reached approximately 2-4 hours after ingestion of lithium carbonate. Lithium plasma levels are usually monitored at least twice weekly in order to maintain this level within the range of from about 0.5 to about 1.0 mmol/L; for severe cases of mania, however, the lithium plasma level may be increased to about 1.5 mmol/L. Therapeutic doses of lithium can cause fatigue, muscular weakness, slurred speech, atoxia, tremor of the hands, nausea, vomiting, diarrhea and thirst. At plasma levels above about 2.2 mmol/L, more serious toxic effects occur, with the CNS primarily affected, i.e., consciousness is impaired, coma may occur, muscular rigidity, hyperactive deep reflexes and marked tremor and muscular fasciculations are observed, epileptic seizures can occur, and EEG abnormalities are common. A lithium plasma level of about 5.0 mmol/L can be fatal. Accordingly, it is not only important, but essential, to know what such plasma level is in order to avoid the deleterious effects associated therewith.
Analysis of lithium in biological fluids (e.g. sera, plasma, urine, cerebro-spinal fluid, or whole blood) is hampered by the presence of other ionic compounds in such fluids, and in particular, sodium ions. This interference is most noticeable at lower lithium concentrations (for example, about 0.10 mmol/L). With respect to serum, the molar ratio of lithium to sodium is about 1:1500. Accordingly, one of the challenges facing those developing an assay method for serum lithium is to overcome the interference from such ions.
Flame photometry is one method for lithium analysis in clinical specimens. In flame photometry, atoms are excited to an energy level above their ground state by a flame. Upon return to ground state, the energy is released as radiation at a frequency characteristic of the element under investigation. By measuring the emission light intensity, the concentration of the analyte of interest in the sample can be determined. Despite its relative simplicity, flame photometry is a tedious procedure and includes several critical disadvantages as an analytical method, e.g., spectral interference between two or more substances in a sample (such as is the case with serum sodium and lithium), background interferences, anionic and cationic interferences, and self-absorption. Furthermore, routine maintenance of the instrumentation is not only critical to ensure good analytical results, but is itself a tedious procedure. Additionally, flame photometry instruments utilize air compressors, which on the whole are noisy, a distinct disadvantage in a clinical setting. Finally, there is the practical concern of safely storing a flammable gas in a laboratory environment.
Ion selective electrode (ISE) technology is an alternative to flame photometry which avoids many of these problems. ISE technology involves the use of a reference electrode and an ion selective electrode separated by a membrane. The ion selective electrode is specific or sensitive to the particular ion of interest. Typically, the reference electrode and the ion selective electrode are simultaneously immersed into a sample solution. An electrical potential is developed between the electrodes which is relative to the presence of the ion to which the ISE is sensitive or specific. This potential can be utilized to determine the concentration of that ion in the sample. Most often the investigator desires to only measure the concentration of one ion out of the many different ions in solution. Thus, the ion selective composition of the ISE, referred to as a "carrier" or "ionophore", must be capable of sequentially complexing the desired ion, transporting the complexed ion across the membrane, and releasing the ion, in preference to all other ions which may be present in the sample solution.
Macrocyclic polyethers, also referred to as cryptanols or "crown ethers", are well known lithium ion-complexing compounds which are suitable for use as ion selective electrodes. Such ionophores are described in, for example, U.S. Pat. No. 4,214,968; U.S. Pat. No. 4,504,368; Oesch, U. et al., "Ion Selective Membrane Electrodes for Clinical Use", Clin. Chem. 32:1448-1459 (1986); Kitayama, S. et al. "Lithium-Selective Polymeric Electrodes Based on Dodecylmethyl-14-Crown-4", Analyst 110:295-299 (1985); and, Attiyat, A. et al., "Comparative Evaluation of Neutral and Proton-Ionizable Crown Ether Compounds as Lithium in Ion-Selective Electrodes and in Solvent Extraction", Anal. Chem. 60:2561-2564 (1988). Other lithium ion complexing compounds are also available, and examples of these are described in U.S. Pat. No. 4,853,090; Gadzekpo, V. P. Y. et al., "Lipophilic Lithium Ion Carrier in a Lithium Ion Selective Electrode", Anal. Chem. 57:493-495 (1985); and, Gadzekpo, V.P.Y. et al., "Problems in the Application of Ion-Selective Electrodes to Serum Lithium Analysis", Analyst, 111:567-570 (1986). All of the preceding references of this paragraph are incorporated herein by reference.
Two methods are associated with ISE: the direct potentiometric method, where the sample is measured directly; and the indirect potentiometric method, where the sample is diluted prior to analysis. Of the two, the indirect potentiometric method is preferred because: (a) dilution of the sample derives the advantages of mass action law, (b) serum results do not usually correlate well between flame photometry and the direct potentiometric method, and (c) maintenance of a direct ISE analyzer is more difficult than an indirect ISE due to protein build-up on the direct ISE. However, despite the advantages of the indirect potentiometric methodology, there are no indirect potentiometric analyzers for lithium commercially available. This is based, in part, on the relationship of lithium in a clinical specimen to other ions therein. For example, in serum, where as previously noted the ratio of lithum to sodium is 1:1500, dilution of a serum sample makes analysis of the already miniscule amount of lithium present very difficult.
Thus, a need exists for an indirect ISE method for the determination of lithium in clinical samples. Additionally, because of the need to dilute the clinical sample for the indirect potentiometric analysis thereof, the diluent utilized is also important for proper analysis of lithum in clinical samples. Accordingly, a need also exists for a diluent useful in the indirect potentiometric determination of lithium in clinical samples.